This invention relates generally to measuring electrical current such as in semiconductor device testers, and more particularly the invention relates to the use of a current range finder to expedite the selection of current sub-ranges for testing.
Measuring electric current (hereinafter “current”) accurately is a key requirement for all parametric testers, in particular those used for semiconductor devices characterization (“SPT” for “Semiconductor Parametric Tester”). The wide range of semiconductor devices and applications implies measured current levels as low as 1e–12 A (pA) and as high as 1 A, namely many orders of magnitude or decades of current. This leads to optimization techniques, in which the entire range of current is divided into several sub-ranges, where the critical measuring components are switched in and out automatically during measurement, until the best sub-range is found and the measured data acquired.
A typical example is the Output Characteristics of an active device, such as a Bipolar or MOS transistor. A constant stimulus (voltage for MOS, current or voltage for Bipolar) is applied to the control pin (gate, base, respectively), while another stimulus (voltage) is applied between the output pin (drain, collector, respectively) and the common pin (source, emitter, respectively), and varied sequentially by equal steps from minimum to maximum values, with the respective output current measured accordingly. After the sequence is complete the stimulus at the control pin is stepped to another constant value, and the output sequence repeated until a “family” of output sequences is generated, each corresponding to constant stimulus at the control pin (referred to as “parameter”). Stepping from one point to the next, along a sequence, does not usually require a new sub-range, with only a small number of points requiring a single sub-range change. However, the transition from the last point of one sequence to the first point of the next sequence often requires many such changes until the optimized sub-range is reached. This takes more time, as changing sub-range requires additional time delays to assure “glitch-free” transition. As minimizing the overall measurement time is commonly required, reducing these delays is more than desirable.
The present invention provides an efficient way to reduce the delay time due to multiple sub-range changes, by practically assuring a single sub-range change regardless how many sub-ranges separate between the last measured point and the new one. More specifically, considering an added time delay “T” per sub-range change, the total related time delay when “n” such changes are required to reach the optimized sub-range is nT. In contrast, using this invention the time delay is just T; namely reducing measurement time for such step by nT−T=(n−1)T.
The present invention provides an effective method of selecting the current sub-range for measurement.